Basic mechanisms of visual spatial attention revealed by mental curve tracing and the dynamics of electrical brain activity

Mental curve tracing is the process by which a contour is covertly followed between two landmarks. Completion time of this task increases as the distance between the landmarks does, even though the Euclidian distance is constant. This has been taken as evidence that attention does not cover a contour instantly, but rather moves from one point to another until the whole contour has been covered. This article provides an electrophysiological measurement of the time course of this spread of attention in humans using a sustained contralateral posterior negative (SPCN) eventrelated potential component. This component being elicited only when stimuli are presented laterally, the position of lateralization was varied to modulate the onset of this SPCN. Curves that became lateralized further from the central starting point yielded a later SPCN onset than curves that lateralized nearer. This provides converging evidence that attention moves along the curve.